A metal oxide, such as Indium Tin Oxide (ITO), has been used in transparent conductive films that need to have a high light transmittance, such as the transparent conductive film provided on the display surface of a touch panel or other display panel and the transparent conductive film in an information input device provided on the display surface of a display panel. However, the manufacturing cost of a transparent conductive film using a metal oxide is high, since the film is formed by sputtering in a vacuum environment. Furthermore, such a film tends to crack or peel due to deformation such as bending or warping.
Therefore, as an alternative to a transparent conductive film using a metal oxide, a transparent conductive film using metal nanowires is being studied. This film may be formed by coating or printing and is also highly resistant to bending and warping. Such a transparent conductive film using metal nanowires is also attracting attention as a next-generation transparent conductive film that eliminates the use of indium, which is a rare metal (for example, see JP 2010-507199 A (PTL 1) and JP 2010-525526 A (PTL 2)).
The transparent conductive film described in PTL 1, however, might exhibit redness and suffer a loss of transparency.
Furthermore, when the transparent conductive film using metal nanowires is provided on the display surface of a display panel, external light is diffusely reflected by the surfaces of the metal nanowires, causing a “black float phenomenon” where the display of black by the panel is slightly bright. The black float phenomenon lowers the contrast of images, thereby deteriorating display characteristics.
As one approach to prevent the occurrence of black floating, gold nanotubes using Gold (Au), which does not tend to reflect light diffusely, have been proposed. Formation of gold nanotubes starts with application of a gold plating to a template of silver nanowires, which tend to reflect light diffusely. Subsequently, the silver nanowire portions used as the template are etched or oxidized for conversion to gold nanotubes (for example, see JP 2010-525527 A (PTL 3)).
As another approach for preventing the diffuse reflection of light, use of a combination of metal nanowires and secondary conductive media (such as Carbon Nanotubes (CNTs), conductive polymers, and ITO) has been proposed (for example, see PTL 2).
However, with the gold nanotubes obtained with the former approach, the silver nanowires used as a template are a waste of material, and moreover, metal material for the gold plating is required. This increases the cost of the materials and complicates the manufacturing process, resulting in an increased manufacturing cost.
The latter approach runs the risk of a loss of transparency due to the secondary conductive media (coloring materials) such as the CNTs, conductive polymers, and ITO being located in gaps in the metal nanowire network.
To solve the above problems, a transparent conductive film including metal nanowires and a colored compound (dye) adsorbed on the metal nanowires has been proposed (for example, see JP 2012-190777 A (PTL 4) and JP 2012-190780 A (PTL 5)). In the proposed transparent conductive film including the metal nanowires and the colored compound (dye) adsorbed on the metal nanowires, the colored compound adsorbed on the metal nanowires absorbs visible light and prevents diffuse reflection of light at the surfaces of the metal nanowires. In the transparent conductive film, the colored compound (dye) adsorbed on the metal nanowires is represented by, for example, the formula R—X, where R is a chromophore and X is an adsorptive functional group. Accordingly, the loss of transparency due to the addition of the colored compound (dye) is controlled.
In the colored compound (dye) adsorbed on the metal nanowires, however, when the adsorptive functional group X is a sulfo group, a sulfonate group, an amino group, a carboxyl group, a carboxyalte group, or the like, the metal nanowires might dissolve. Therefore, a transparent conductive film (transparent electrode) containing metal nanowires that are surface-modified with a colored compound (dye) is problematic in that the metal nanowires may break and become shorter, and the conductivity may degrade.